This invention relates to the polishing of semiconductor wafers of the type from which chips for integrated circuits and the like are made and, more particularly, to a polishing head for positioning such a wafer for the polishing of a face thereof by a procedure which includes engagement of the face with a polishing surface, and a method of such polishing.
Integrated circuits typically are provided as "chips", each of which includes a piece of flat material that has the desired circuitry. Typically, a multiple number of the desired integrated circuits are formed at the same time by etching and coating a disk-shaped semiconductor wafer substrate. This wafer is then diced into flat pieces which are individually provided with suitable packaging having the leads necessary to electrically access the integrated circuitry. These packaged pieces of material (dies) are referred to as integrated circuit chips. In certain instances, a full wafer is used to form a single integrated circuit rather than duplicates of a desired integrated circuit.
The disk-shaped wafer substrates typically are comprised of a monocrystalline semiconductor, such as single crystal silicon. One common method of forming the wafers is to grow a relatively long cylinder or log of a single crystal of the material, and then slice the log (often called a boule) to form the individual disk-shaped wafers. It should be noted that while by far the greatest use of semiconductor wafers is as substrates for integrated circuitry, there are other uses, e.g., as solar cells.
It is necessary for the formation of various circuits or for other uses of wafers, that the active or front face, e.g., the face of the wafer on which the integrated circuitry is to be formed, be highly polished. (The other side of the wafer is often referred to as the wafer "back" face.) To this end, polishing machines have been designed to provide the desired finish. These machines typically bring the face of the wafer to be polished into engagement with a treating surface, such as the polishing surface of a rotating polishing pad having a desired polishing material, e.g., a slurry of colloida silica, applied thereto. In many instances, the polishing head which holds the wafer with the face exposed also rotates. It is the movement between the wafer and the polishing pad which results in the desired polishing. In some instances this "polishing" is provided primarily for the purpose of making one face flat, or parallel to another face. In this connection, it must be remembered that the wafer itself is monocrystalline, and characteristics of this type may be quite important in making the same suitable for the production of integrated circuitry or for some other desired use.
It will be recognized that the engagement of the face of the wafer and the polishing pad moving relative thereto will result in a lateral force being applied to the wafer tending to move the same in an uncontrolled manner. It is desirable that there be no uncontrolled movement, though, to provide the degree of control to the polishing process that is required. This problem is particularly acute in machines designed to polish a multiple number of wafers at the same time, as opposed to providing single wafer polishing. Reference is made, for example, to U.S. Pat. No. 4,918,870, which describes the use of "floating" subcarriers to enable the benefits of single wafer polishing to be achieved with the economies of multiple wafer polishing.
Carriers for wafers to be polished often have included wafer-holding inserts and/or retainers which define pockets for holding the wafers in a set position. The difficulty is that such an insert or retainer interferes with the polishing of the exposed wafer face. The insert or retainer is itself held against the polishing pad, and although it holds the wafer in a desired position its own surface and thickness characteristics deleteriously affect the wafer face polishing.